The present invention relates to radiation therapy systems for the treatment of cancer and the like and, in particular, to a system providing improved treatment speed and accuracy.
External beam radiation therapy may treat a tumor within the patient by directing high-energy radiation in one or more beams toward the tumor. Recent advanced external beam radiation systems, for example, as manufactured by Tomotherapy, Inc., treat a tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated beamlets whose intensities can be controlled so that the combined effect of the beamlets, over the range of angles, allows an arbitrarily complex treatment area to be defined.
X-rays deposit energy in tissue along the entire path between the x-ray source and the exit point in the patient. While judicious selection of the angles and intensities of the x-ray beamlets can minimize radiation applied to healthy tissue outside of the tumor, inevitability of irradiating healthy tissue along the path to the tumor has suggested the use of ions such as protons as a substitute for x-ray radiation. Unlike x-rays, protons may be controlled to stop within the tissue, reducing or eliminating exit dose through healthy tissue on the far side of the tumor. Further, the dose deposited by a proton beam is not uniform along the entrance path of the beam, but rises substantially to a “Bragg peak” near a point where the proton beam stops within the tissue. The placement of Bragg peaks inside the tumor allows for improved sparing of normal tissue for proton treatments relative to x-ray treatments.
Current proton therapy systems adopt one of two general approaches. In the first approach, the proton beam is expanded to subtend the entire tumor and the energy of the protons, and hence their stopping point in the tissue, is spread in range, to roughly match the tumor depth. Precise shaping of the exposure volume is provided by a specially constructed range correction compensator which provides additional range shifting to conform the distal edge of the beam to the distal edge of the tumor. This treatment approach essentially treats the entire tumor at once and, thus, is fast and yet less precise and requires the construction of special compensators for each treatment field.
In a second approach, termed the “magnetic spot scanning” (MSS) approach, the proton beam remains narrowly collimated in a “pencil beam” and is steered in angle and modulated in range to deposit the dose as a series of small spots within the patient. The spots are located to cover the tumor in successive exposures until an arbitrary tumor volume has been irradiated. This approach is potentially very accurate, but because the tumor is treated in many successive exposures, this approach is much slower than the SOBP approach. Further the small spot sizes create the risk of uneven dose placement or “cold spots” between the treatment spots, something that is exacerbated if there is any patient movement between exposures.
The benefits of both of these techniques, without the drawbacks, might be obtained if it were possible to produce an areal beam composed of individually modulated pencil beams. Producing a treatment beam of this type would require an areal modulator capable of receiving the areal beam and separately modulating small rays within the beam.
An areal modulator intended for x-ray radiation is described in U.S. Pat. No. 5,802,136 to Carol entitled: “Method and Apparatus For Conformal Radiation Therapy” issued Sep. 1, 1998 and hereby incorporated by reference. This modulator employs a chamber positioned within the beam and holding a pool of mercury. Within the mercury are axially extending balloons loosely stabilized by radiolucent pins. When the balloons are deflated, radiation along the beam axis is blocked by the mercury. When the balloons are inflated, each balloon provides a separate channel allowing passage of the radiation. The balloons may be individually inflated and deflated and each deflated balloon is extremely thin and deflates to an undulating membrane so as to prevent leakage of radiation through the deflated balloon (hotspots). Further, the balloons, when fully inflated, may effectively displace mercury between them eliminating cold spots as might be obtained were the balloons separated by rigid radio-opaque walls.
Mercury is relatively heavy and toxic and the accurate control of loosely constrained, flexible balloons in a bath of mercury is a difficult engineering problem. Possibly for this reason, Carol also describes an embodiment in which the balloons are arranged in rigid compartments arranged in “checkerboard” fashion, with a balloon in every other cell of the checkerboard and the remaining cells being radio-opaque. The cold spots generated by the radio-opaque cells are dealt with by making two successive exposures of the patient with the checkerboard shifted appropriately between the exposures, for example, rotated about its axis or used to expose the patient from the opposite side of the patient after rotating about the patient. In this way the opaque cells and the cells with the balloons switch places to provide for complete exposure.